1. Field of the Invention
The present invention relates to a process for producing a granular slow-acting nitrogenous fertilizer. More particularly, it relates to a process for producing a granular slow-acting nitrogenous fertilizer excellent in the fertilizer physical properties and fertilizing effects using an urea-isobutyraldehyde condensate.
2. Discussion of Background
In application of a fast-acting nitrogenous fertilizer, it has been common to employ a method wherein the necessary amount thereof is divided and applied in a few times in order to avoid adverse effects of high nitrogen concentration to plants or to improve the nitrogen utilization efficiency. Whereas, a slow-acting nitrogenous fertilizer such as a urea-isobutyraldehyde condensate (e.g. isobutylidene diurea) is hardly soluble in water, and it is gradually decomposed to inorganic nitrogen in soil and utilized by plants in the form of inorganic nitrogen, whereby adverse effects to plant due to high nitrogen concentration are scarcely brought about, and the necessary amount can be applied all at once. Further, it has a merit that the utilization efficiency by plants is high, since leaching or run-off scarcely occurs. Thus, the slow-acting nitrogenous fertilizer such as an isobutylidene diurea is an excellent fertilizer in that the application is easy as compared with a fast-acting nitrogenous fertilizer, it is economically advantageous that the utilization efficiency by plants is high, and it is thereby possible to prevent environmental pollution.
As a method for preparing this isobutylidene diurea, it is known to react, for example, 2 mols of solid urea and 1 mol of isobutyraldehyde using an inorganic acid as a catalyst in a substantially sealed reactor under stirring while maintaining the urea in the solid phase (U.S. Pat. No. 3,870,755). This method is industrially advantageous, since the reaction product is obtainable in the form of powder particles (particle size: fine powder to a few mm). However, it has a drawback that a urea-isobutyraldehyde condensate powder having a particle size of not larger than 1 mm is produced in a substantial amount. The urea-isobutyraldehyde condensate having a small particle size has the following drawbacks. Namely, the smaller the particle size, the larger the hydrolyzing rate, and the more readily it is decomposed to inorganic nitrogen in soil, whereby the function as a slow-acting nitrogenous fertilizer can not adequately be performed. Further, the urea-isobutyraldehyde condensate having a small particle size brings about disadvantage in handling such that the flowability is poor, dusting or scattering is likely to take place at the time of application, and the hydrophilic nature is low and its application to water is difficult.
Japanese Examined Patent Publication No. 57845/1986 discloses a method for producing a urea-C.sub.4 saturated aliphatic aldehyde condensate powder particles whereby formation of a powder having a small particle size is suppressed. This publication discloses a method wherein solid urea, a certain specific amount of a C.sub.4 saturated aliphatic aldehyde and a certain specific amount of formaldehyde are supplied continuously to a reactor, and the urea, the C.sub.4 saturated aliphatic aldehyde and the formaldehyde are reacted in the presence of an inorganic acid.
Further, Japanese Examined Patent Publication No. 4935/1983 discloses a method for producing a urea-C.sub.4 saturated aliphatic aldehyde condensate which comprises continuously supplying solid urea to a reactor and reacting the urea in the solid phase with a C.sub.4 saturated aliphatic aldehyde in the presence of an inorganic acid, wherein a urea-formaldehyde condensate polycondensable under an acidic condition is supplied to the reactor. By these methods, it is possible to suppress formation of a powder having a particle size of less than 1 mm to some extent. However, the rate of formation is still as high as from about 15 to 50%, which is not satisfactory.
On the other hand, Japanese Unexamined Patent Publication No. 26585/1986 relates to an aggregate fertilizer comprising (A) a hardly soluble granular fertilizer and (B) a solidified product obtained by reacting urea and formaldehyde in a formalin solution having urea dissolved therein, wherein said hardly soluble granular fertilizer is dispersed and contained in said solidified product. Specifically, it discloses a method wherein, for example, a coated ammonium nitrate potash fertilizer covered with concentrated phospholic acid, is filled in an ice-forming tray, then a formalin solution having urea dissolved therein under heating, is poured, and the addition condensation reaction of urea and formaldehyde is conducted to obtain an aggregate fertilizer of a truncated quadrangular pyramid shape. However, the hardly soluble granular fertilizer as the starting material of such an aggregate fertilizer is required to have a particle size so that when the ureaformalin solution is mixed with the granular fertilizer, the solution should be able to pass through spaces among particles to contact all the particles uniformly. Specifically, the particle size is required to be about 1 mm or larger, and there is no disclosure of using a urea-C.sub.4 saturated aliphatic aldehyde condensate of a powder form as the starting material. Further, said publication relating to an aggregate fertilizer does not disclose a so-called reaction granulation wherein granulation is conducted while reacting the starting materials.
On the other hand, with respect to the granulation of urea and a C.sub.4 saturated aliphatic aldehyde condensate powder or small granules, Japanese Examined Patent Publication No. 22899/1979 discloses a method for producing a slow-acting granular nitrogenous fertilizer, which comprises heating and granulating a powder or small granules of a condensate of urea with a C.sub.4 saturated aliphatic aldehyde in the presence of an inorganic acid and a C.sub.4 saturated aliphatic aldehyde. This method is an effective method for granulating a powder or small granules of a condensate of urea with a C.sub.4 saturated aliphatic aldehyde. However, the C.sub.4 saturated aliphatic aldehyde is inflammable, and the reaction is required to be conducted in a closed system. Thus, the method requires a cumbersome means, and the granular fertilizer thereby obtained is not fully satisfactory with respect to the fertilizer physical properties such as the hardness, particle size and shape of the granules.
The present invention has been accomplished in view of the above circumstances. It is an object of the present invention to provide a process for producing a granular slow-acting nitrogenous fertilizer having sufficient hardness by using a powder which is produced in a large amount during the production of a urea-isobutyraldehyde condensate, as the starting material.
The present inventors have conducted a study to convert a urea-isobutyraldehyde condensate powder to a useful slow acting nitrogenous fertilizer. As a result, it has been found that when water is used as a binder, as is commonly employed for a usual granular fertilizer, the granulated product is inadequate in the hardness and brittle and thus has a drawback that it readily disintegrates into a powder, whereas when methylol urea is used as a binder and granulation is conducted while subjecting the methylol urea to dehydration condensation, it is possible to obtain a granular fertilizer which is extremely useful as a slow-acting fertilizer having sufficient hardness and an adequately slow rate for conversion to inorganic nitrogen.
Thus, the present invention provides a process for producing a granular slow-acting nitrogenous fertilizer, which comprises blending an aqueous methylol urea solution to a urea-isobutyraldehyde condensate powder or a mixture thereof with a urea-formaldehyde condensate powder, and granulating the resulting blend while subjecting the methylol urea to dehydration condensation under an acidic condition to form a urea-formaldehyde condensate.